A thermoelectric conversion material is a material that is capable of converting thermal energy directly into electricity or electrical energy directly into thermal energy, and, in other words, is capable of heating and cooling by applying electricity. A thermoelectric conversion module is formed by electrically connecting in series a large number of pairs of p/n thermoelectric conversion materials, each of which is a combination of a p-type thermoelectric conversion material and an n-type thermoelectric conversion material. With the use of a thermoelectric conversion module, waste heat, which has not been used often, can be converted into electricity to make effective use of energy.
The performance of a thermoelectric conversion material is assessed by its figure of merit Z. The figure of merit Z is expressed by the following formula (1), in which a Seebeck coefficient S, thermal conductivity κ, and electric resistivity ρ are used:Z=S2/(κρ)  (1)
The performance of a thermoelectric conversion material is also assessed by the product of its figure of merit Z and temperature T. In this case, both sides of the formula (1) are multiplied by the temperature T (T being absolute temperature) to obtain the following formula (2):ZT=S2T/(κρ)  (2)
ZT in the formula (2) is called a dimensionless figure of merit, and serves as an indicator of the performance of the thermoelectric conversion material. A thermoelectric conversion material with a larger ZT has better thermoelectric performance at the temperature T. According to the formulas (1) and (2), an excellent thermoelectric conversion material is a material that is capable of increasing the value of the figure of merit Z, that is a material that has a larger Seebeck coefficient S, low thermal conductivity κ and small electric resistivity ρ.
Also, when the performance of a thermoelectric conversion material is assessed from an electrical point of view, the electrical power factor P expressed by the following formula (3) is used in some cases:P=S2/ρ  (3)
The highest conversion efficiency ηmax of a thermoelectric conversion material is expressed by the following formula (4):ηmax={(Th−Tc)/Th}{(M−1)/(M+1)/(Tc/Th))  (4)
M in the formula (4) is expressed by the following formula (5), where Th represents the temperature of the hot side of the thermoelectric conversion material, and Tc represents the temperature of the cold side of the thermoelectric conversion material:M={1+Z(Th+Tc)/2}−0.5  (5)
According to the above formulas (1) to (5), it is known that the higher the figure of merit and the temperature difference between the hot side and the cold side, the larger the thermoelectric conversion efficiency of a thermoelectric conversion material.
Typical examples of thermoelectric conversion materials to be used in thermoelectric conversion modules that have been studied thus far include Bi2Te3-based materials, PbTe-based materials, AgSbTe2—GeTe-based materials, SiGe-based materials, (Ti, Zr, Hf) NiSn-based materials, CoSb3-based materials, Zn4Sb3-based materials, FeSi2-based materials, B4C-based materials, NaCo2O4-based oxides, and Ca3Co4O9-based oxides. However, only Bi2Te3-based materials among these materials have been put into practical use. The highest working temperature of the thermoelectric conversion modules containing Bi2Te3-based thermoelectric conversion materials used for power generation is limited to 250° C., which is the highest temperature the Bi2Te3-based materials can endure.
To effectively use various kinds of waste heat, there has been a demand for thermoelectric conversion modules that can be used at intermediate temperatures ranging from 300 to 600° C. In recent years, attention is drawn to a filled skutterudite thermoelectric conversion material that can be used in this temperature range. A filled skutterudite compound is expressed by the chemical formula RT4X12 (R being a metal, T being a transition metal, X being pnictogen), and has a cubic structure of the space group Im−3. In this formula, R represents an alkaline-earth metal, a lanthanoid, or an actinoid, T represents a transition metal such as Fe, Ru, Os, Co, Pd, or Pt, and X represents a pnictogen element such as As, P, or Sb. Particularly, studies are being actively made on filled skutterudite thermoelectric conversion materials having Sb as X.
La(Ce)—Fe—Sb or Yb—Co—Sb skutterudite thermoelectric conversion materials that have been developed, particularly, p-type CeFe4Sb12, p-type LaxFe3CoSb12 (0<x≤1), and n-type YbyCo4Sb12 (0<y≤1) thermoelectric conversion materials, have relatively good thermoelectric performance at intermediate temperatures ranging from 300 to 600° C. As for such thermoelectric performance, it is described that the dimensionless figure of merit ZT is 0.9 to 1.4 in Patent Document 1, and that is 0.7 to 0.8 in Patent Document 2.
However, to produce a thermoelectric conversion module having high thermoelectric conversion efficiency, a thermoelectric conversion material having a higher dimensionless figure of merit ZT over a wide temperature range is required. When the inventors conducted an additional test based on Patent Document 1, the dimensionless figure of merit ZT of the p-type CeFe4Sb12 thermoelectric material disclosed in Patent Document 1 was 0.5 to 0.6 at 450° C. Therefore, the dimensionless figure of merit ZT did not reach the ZT value of 1.4 disclosed in Patent Document 1. On the other hand, the dimensionless figure of merit ZT of an n-type thermoelectric material being able to be used at temperatures ranging from room temperature to 600° C. is 0.5 at 200° C., 0.6 at 300° C., and 0.8 at 500° C. Therefore, the value of the dimensionless figure of merit ZT indicating the thermoelectric performance is small at temperatures ranging from room temperature to 600° C., particularly ZT is much smaller at temperature below 300° C. As described above, with the conventional materials, it is difficult to achieve a higher dimensionless figure of merit ZT over a wide temperature range.
Meanwhile, to produce thermoelectric conversion modules that can be used at intermediate temperatures ranging from 300 to 600° C., selecting the electrode material to connect a p-type thermoelectric conversion material and an n-type thermoelectric conversion material, and joining the materials are crucial tasks. It is essential that good junction properties are maintained between the electrode material and the thermoelectric conversion materials, and the properties of the thermoelectric conversion materials do not deteriorate due to the electrode material. To realize this, consistency in thermal expansion coefficient among the thermoelectric conversion materials, the electrode material and the material used for joining them, and stability of the joining layers at the joint interfaces over an operating temperature range up to 600° C. are essential. If the difference in thermal expansion coefficient is large, a large thermal stress is generated there, disadvantageously causing breaking at the joining portions. Also, if element diffusion progresses at the joint interfaces between the electrode material and the thermoelectric conversion materials, the thermoelectric properties deteriorate, and the performance of the electrode material becomes poorer.
To address the above problems, it is disclosed in Patent Document 3 that a layer of titanium or a titanium alloy is provided between thermoelectric conversion materials and an electrode material at a high-temperature portion of the thermoelectric conversion material of a skutterudite structure.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2000-252526    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2001-135865    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2003-309294
When the inventors conducted an additional test using a filled skutterudite thermoelectric conversion material and Ti as a joining material, however, the electrode material separated from the filled skutterudite thermoelectric conversion material. One of the causes of the separation is considered to be due to the fact that the difference between the thermal expansion coefficient of the thermoelectric conversion material and the thermal expansion coefficient of the electrode material became larger with an increase in temperature, particularly at temperatures of 400° C. or higher, and a thermal stress was generated.